Currently, most wastewater treatment plants use an activated sludge process, based on biological oxidation of organic contaminants in a suspended growth medium. Oxygen is supplied from air using bubble type aerators. Efficiency of these systems is poor resulting in very high energy use. Tank size is large as chemical oxygen demand loadings are low because of low biomass concentration. The result is high capital and operating cost.
A second type of established biological oxidation process uses biofilms grown on a media. The wastewater is circulated to the top of the reactor and trickles down. Air is supplied at the bottom. The rate of oxygen transfer is limited by the biofilm surface area, and the operating cost is high because of wastewater pumping requirements. Other versions of this process are also available, but all of these result in high operating costs.
Recently, development work has been done on a membrane supported bioreactor concept. This process involves growing biofilm on the surface of a permeable membrane. Oxygen containing gas is supplied on one side of the membrane and the biofilm is grown on the other side, which is exposed to the substrate. Oxygen transferred through the membrane is absorbed by the biofilm as it is available in the form of very fine bubbles. This type of process has not become commercially viable.